For example, in a polymer electrolyte fuel cell, a cell is formed as a minimum unit by sandwiching a membrane electrode assembly (MEA), which, as illustrated in FIG. 8, is composed of a fuel electrode 70 and an air electrode 74 sandwiching an electrolyte membrane 72 formed of a polymer electrolyte membrane, with two separators 80, and a plurality of cells are normally stacked to form a fuel cell stack (an FC stack), which can provide a high voltage.
As the mechanism for generating electricity by a polymer electrolyte fuel cell, generally, fuel gas, such as hydrogen-containing gas, is supplied to the fuel electrode (i.e., anode-side electrode), and oxidant gas, such as gas mainly containing oxygen (O2) or air, is supplied to the air electrode (i.e., a cathode-side electrode). The hydrogen-containing gas is supplied to the fuel electrode 70 through a fuel gas flow channel, and is decomposed into electrons and hydrogen ions (H+) by an action of a catalyst of the electrode. The electrons move from the fuel electrode 70 to the air electrode 74 through an external circuit and produce electric current. Meanwhile, the hydrogen ions (H+) pass through the electrolyte membrane 72 to reach the air electrode 74, where the hydrogen ions bond to oxygen and the electrons which have passed through the external circuit, to thereby produce reaction water (H2O). The heat generated simultaneously with the bonding reaction of hydrogen (H2) to oxygen (O2) and the electrons is collected by cooling water. Further, water generated on the cathode side where the air electrode 74 is present (which will be hereinafter referred to as “generated water”) is drained from the cathode side.
The fuel electrode and the air electrode of a fuel cell described above are formed of catalyst layers which include a stack of gas diffusion layers for diffusing hydrogen-containing gas and oxidant gas, respectively. Here, if the drainage of the generated water generated by the above-described reaction is interrupted on the cathode side, there is a possibility of occurrence of a clogging phenomenon (which is also referred to by “a flooding phenomenon”). In order to address this problem, the gas diffusion layer is generally formed of a layer made of carbon fibers and a water repellent layer, and prevents the flooding phenomenon by facilitating drainage of the generated water.
However, if at least a portion of the carbon fibers in the gas diffusion layer protrudes, this protruding portion of the carbon fibers may damage the membrane-electrode assembly when the gas diffusion layers are stacked to form the membrane-electrode assembly.
Patent Literature 1 suggests a gas diffusion layer material for a fuel cell, which has a smooth surface and which is thin, obtained by passing a fabric or non-woven fabric including flame-resistant threads for carbon fiber as main components through a thermo-compression roller to smooth the surface and reduce the thickness thereof, and thereafter subjecting the resultant fabric or non-woven fabric to final thermal processing at temperatures of 800 to 3000° C. Further, Patent Literature 2 suggests pre-heating the surface of a gas diffusion layer for a fuel cell, which is formed of a fabric composed of warp and weft made of carbon fiber, to thereby smooth the uneven surface of the carbon fiber, prior to disposing the gas diffusion layer on a polymer electrolyte membrane.
Meanwhile, Patent Literature 3 suggests a method for placing a gas diffusion layer base including fibers on a roller to curve the gas diffusion layer base and cause fiber protrusions protruding from the gas diffusion layer base to rise, and removing the fiber protrusions. Patent Literature 4 suggests a method of manufacturing a water-repellent gas diffusion layer having a two-layer structure, by using two types of water-repellent pastes having a conductive material and a water-repellent agent.    Patent Literature 1: JP 2004-111341 A    Patent Literature 2: Pamphlet of WO2003/081700    Patent Literature 3: JP 2008-198526 A    Patent Literature 4: JP 2009-181891 A